1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method for producing an image by forming dots having two or more sizes on a recording medium.
2. Description of the Related Art
With the significant advances in the inkjet technology in recent years, it has been becoming possible for inkjet printers to produce large color prints of high quality at high speeds. The inkjet printers are widely used particularly in sign and display applications, and are applicable to the printing of POP (Point Of Purchase) posters, wall posters, outdoor advertisements, and billboards, etc., for example. The inkjet printers are capable of producing prints by ejecting droplets of a plurality of types of inks, e.g., inks in C (cyan), M (magenta), Y (yellow), and K (black), onto a print medium (recording medium) to form a number of dots thereon.
Attempts made in connection to the inkjet printers to speed up the formation images and increase the density of dots tend to cause problems due to an interference between landed ink droplets. The interference between landed ink droplets refers to a phenomenon wherein in a state where an earlier droplet that has previously landed on a recording medium has not yet been fully absorbed in the recording medium, a subsequent droplet lands on the recording medium near the unabsorbed portion of the earlier droplet and unites the earlier droplet. If the interference between landed ink droplets occurs, the liquid inks of the droplets shift toward each other due to their surface tension. As a consequence, the position of the center of gravity of a dot formed on the recording medium by the subsequent droplet is shifted, resulting in a reduction of the quality of an image formed on the recording medium.
Accordingly, various inkjet printers have been proposed to form high-quality images by appropriately controlling a recording head (dot forming unit) to eject ink droplets in view of adverse effects of the interference between landed ink droplets.
Japanese Laid-Open Patent Publication No. 2005-313635 discloses a method of and an apparatus for controlling ejection of ink droplets such that the ink droplets are ejected successively from a recording head toward a recording medium in progressively greater quantities. The ink droplets that are ejected in progressively greater quantities from the recording head are effective to form a line having a uniform width on the recording medium even in the event of an interference between landed ink droplets.